There is a part in the last dungeon in the 4th disk... you will be in a place that looks like a "bog" (if you can call it that) its full of magic pots and little red ball things that give a ton of AP!! They usually come in partys of 3 or 4 and are extra weak... make sure you have a weapon that has growth (not like clouds ultima weapon) and when you kill the little guys you should get 2400AP a battle... equip all materia and master it to make big bucks (all materia is the easiest to master)